1. Field of the Invention
This invention relates to a coating apparatus, and particularly to a coating apparatus which is capable of performing an uniform coating on a support.
2. Description of Related Art
Various kind of coating methods such as a roll coating method, a gravure coating method, a slide bead coating method and a doctor coating method have been conventionally used as a method of applying a coating liquid onto a support. However, an extrusion coating method has recently come to public attention as it provides a high productivity and a high operating performance, as well as an outstanding controllability of the thickness of a coated film on the support.
In the extrusion coating method, a coating operation is performed as follows. A front edge (at the upstream side of a support feeding direction) and a back edge (at the downstream side of the support feeding direction) of an extrusion coating head are pressed against a support in the middle of feeding means such as support rolls or the like so that a prescribed tension is applied to the support. In this case, the clearance between the back edge surface and the support is controlled to adjust the thickness of a coated film to a desired value while changing an extrusion amount of coating liquid which is extruded through a slit portion.
In the extrusion coating method as described above, the back edge of the extrusion coating head serves as a doctor edge, and thus the state of the end portion of the back edge at the downstream side of the support feeding direction (i.e., the back end edge) has an important effect on a coating surface characteristic. Therefore, the back end edge is required to be sharp. For example, there has been proposed a coating apparatus having a back end edge which is designed to be right-angled in section (as disclosed in Japanese Laid-open Patent Application No. 52-22039).
However, when a grinding work is conducted on the back end edge to sharpen the back end edge, the grinding work necessarily induces occurrence of burr in the back end edge, and this burr finally causes a problem that streaks occur on a coated film.
Furthermore, there has been proposed a coating apparatus in which a straightness of a front edge surface and/or a straightness of a back edge surface is set to 50 .mu.m or less, preferably 5 .mu.m or less (as disclosed in Japanese Laid-open Patent Application No. 2-207866). However, in the coating apparatus as disclosed in Japanese laid-open Patent Application No. 2-207866, a straightness of a back end edge which acts on a coat surface characteristic, a thickness of a coated film, etc. is not set.
When a burr removing work (burring work) is conducted on the back end edge to remove the burr as described above, it is difficult to control R-surface, C-surface or chamfering work in .mu.m order which is accompanied by the burring work. Therefore, the straightness of the back end edge which acts on the coat surface characteristic, the thickness of the coated film, etc. is reduced, and the thickness of the coated film in a transverse direction greatly varies, so that it is difficult control the thickness of the coated film. Furthermore, even if the back end edge could be processed in .mu.m order, the back end edge will be worn when coating liquid containing particles having relatively high hardness, such as magnetic coating liquid, is used. If such a worn portion of the back end edge is repetitively ground again to finish the back end edge, the precision of the finishing work trends to be gradually reduced as the frequency of the grinding work increases.
In addition, the importance of the back edge serving as the doctor edge has been known as described above, however, a working method and a numerical range for obtaining an excellent coating surface characteristic has been still unclear. Therefore, a judgment on practical use of a produced extrusion coating head can not be made unless the coating operation is actually carried out using tile extrusion coating head, and thus a coating apparatus which is capable of making the judgment on practical use more reasonably has been required.